The main characteristic of selective laser melting (SLM) are summarized below with reference to FIG. 1, which shows a conventional device for fabricating a part by selective melting or selective sintering of powder beds by means of a laser beam.
A first layer 10 of a powder material is deposited on a fabrication plate 80, e.g. with the help of a roller 30 (or any other deposition means), which plate may comprise a plate on its own or may be surmounted by a solid support, by a portion of some other part, or by a support grid used for facilitating building of certain parts.
The powder is transferred from a feeder vessel 70 during a go movement of the roller 30 and then it is scraped and possibly lightly compacted during one (or more) return movements of the roller 30. The powder is made up of particles 60. Excess powder is recovered in a recycler vessel 40 situated adjacent to a building vessel 85 in which the fabrication plate 80 is vertically movable.
Use is also made of a generator 90 for generating a laser beam 95 and of a control system 50 suitable for directing the laser beam 95 onto any region of the fabrication plate 80 so as to scan any region of a layer of previously-deposited powder. The shaping of the laser beam 95 and the varying of its diameter in the focal plane are performed respectively by means of a beam expander 52 and of a focusing system 54, these items together constituting the optical system.
Thereafter, a region of the first layer 10 of powder is raised to a temperature higher than the melting temperature TM of the powder by scanning the laser beam 95.
The SLM method may use any high energy beam instead of the laser beam 95, and in particular it may use an electron beam, providing the beam has sufficient energy to melt the particles of powder and some of the material on which the particles rest (also referred to as the diluted zone and forming an integral portion of the liquid bath).
By way of example, the beam is scanned by a galvanometric head forming part of a control system 50. In non-limiting manner and by way of example the control system comprises at least one steerable mirror 55 on which the laser beam 95 is reflected before reaching a layer of powder, with each point of the surface of that layer being situated at the same height relative to the focusing lens that is contained in the focusing system 54, and with the angular position of the mirror being controlled by a galvanometric head so that the laser beam scans at least a region of the first layer of powder and thus follows a pre-established profile for the part. To do this, the galvanometric head is controlled on the basis of information contained in the database of a computer tool that is used for computer-assisted design and fabrication of the part that is to be fabricated.
Thus, the powder particles 60 in this region of the first layer 10 are melted and form a first single-piece element 15 that is secured to the fabrication plate 80. At this stage, it is also possible to use the laser beam to scan a plurality of independent regions of the first layer so that after the material has melted and solidified a plurality of mutually disjoint first elements 15 are formed.
The fabrication plate 80 is then lowered by a height corresponding to the thickness of the first powder layer 10 (20 micrometers (μm) to 100 μm, and generally 30 μm to 50 μm).
Thereafter, a second powder layer 20 is disposed on the first layer 10 and on the first single-piece or consolidated element 15, and then a region of the second layer 20 that is situated in part or in full over the first single-piece or consolidated element 15 is heated by being exposed to the laser beam 95, as shown in FIG. 1, in such a manner that the powder particles in this region of the second layer 20 are melted together with at least a portion of the element 15 so as to form a second single-piece or consolidated element 25, with these two elements 15 and 25 together forming a single-piece block in the example shown in FIG. 1.
It can be understood that depending on the profile of the part that is to be built up, and in particular when there is an under-cut surface, it can happen that the above-mentioned region of the first layer 10 does not lie, even in part, under the above-mentioned region of the second layer 20, and that under such circumstances, the first consolidated element 15 and the second consolidated element 25 do not form a single-piece block.
This process of building up the part layer by layer is then continued by adding additional powder layers on the unit that has already been formed by the single-piece or consolidated elements 15, 25, . . . .
Scanning with the laser beam 95 makes it possible to build up each layer by giving it a shape that matches the shape of the part that is to be made. The lower layers of the part cool more or less quickly as the upper layers of the part are being built up.
Nevertheless, the SLM method presents drawbacks.
The powder is raised fully above its melting temperature TM by making direct use of the laser beam 95 or by entering into the bath of liquid material heated by the laser beam 95 (indirect melting of the powder). The material of the melted powder is then subjected to a temperature rise and fall cycle as the bath solidifies at its melting temperature and then cools down from TM to ambient temperature.
The bath is heated very quickly since the laser beam 95 delivers a large amount of energy to the material in a very short period of time.
The bath also cools very quickly since heat is thermally pumped from the bath by the solid block of material formed by the layers that have previously been formed under the bath and that have already solidified, and also by the fabrication plate 80.
In addition, in a very short period of time (inversely proportional to the scanning rate of the laser beam 95), the bath passes from an environment that is very hot because of its exposure to the laser beam to an environment that is subjected to a temperature close to ambient, which is equivalent to quenching in air or even to quenching in water.
These successive fast rises and falls in temperature of portions of the part while it is being built up generate stresses and/or deformations depending on the shape and the chocking of the part. The term “chocking” is used to mean the action of using a guide for supporting a thin portion of a part so as to prevent the thin portion deforming.
If the part being built up is solid and therefore not very deformable, stresses accumulate in the part while it is being fabricated, these stresses being in the form of residual stresses or even in the form of cracking when the stresses exceed the breaking stress of the material. Later, in service, if the operating temperature of the part is too high, then the part will deform as a result of such residual stresses relaxing.
If the part being built up presents walls that are thin and with little chocking (i.e. walls for which one dimension is small compared with the other two and that are free to move), the stresses generated during the cooling of each bath deform the part. These deformations lead to a part being fabricated with a final shape that is not the desired shape.
Furthermore, such deformations of the part disturb its fabrication method. Given that the positions of the material strips of a layer depend on the computer-assisted design and fabrication (CADF) file as deduced from the computer-assisted design (CAD) data processing for the part that is to be fabricated and reproducing its volume, there is a risk of an upper layer not being formed completely above a lower layer since the lower layer has been deformed and has moved away from its position as initially specified in the CADF file, for example.
The invention seeks to propose a method that makes it possible to reduce or even eliminate the stresses generated during the formation of the baths that are induced by fast heating and then during the sudden cooling of those baths.
This difficulty is particularly crucial when fabricating superalloy parts that are used in aviation, particularly but not exclusively for low-pressure or high-pressure turbine blades, nozzle parts, turbine ring portions, or combustion chamber portions.
By way of example, use is made of superalloys based on nickel, and in particular of nickel-based superalloys that are reinforced (in particular by adding titanium and/or aluminum) and that make it possible to reach operating temperatures in aviation turbines of the order of 900° C. to 1000° C.
Nevertheless, that type of material is very sensitive to hot cracking: thus, such materials tends to give rise to cracking between grains when the rate of cooling and the temperature gradients are not fully under control.
Certain solutions have already been proposed for overcoming those drawbacks.
In patent EP 1 355 760, the support on which the part is built up is itself heated.
In patent FR 2 856 614, the entire fabrication device is placed in a heated enclosure during the operation of fabricating the part.
In both situations, the idea is to avoid cooling a zone of the part excessively, or more exactly to avoid cooling it too quickly, so as to avoid cracking phenomena.
Nevertheless, those solutions are limited in temperature, since they do not make it possible to exceed 600° C.
Furthermore, those solutions require existing equipment to be greatly modified, by adding additional equipment thereto.